The present invention relates to a process of hydrogenation of the double bonds of the diene units of polymers in the presence of new homogeneous titanium catalysts which are highly soluble in organic solvents and are endowed with a high activity even in the absence of reducing agents. More specifically, the present invention relates to the use of hydrogenation catalysts based on titanium compounds of the type Cp.sub.2 Ti(PhOR).sub.2, where Cp represents a cyclopentadienyl group, Ph a phenyl group, and OR is an alkoxide group containing 1 to 4 carbon atoms and preferentially occupying the 3 or 4 position of the aromatic ring; or on titanium compounds of the type Cp.sub.2 TiR.sub.2, where R is a --CH.sub.2 PPh.sub.2 group, and to the process of hydrogenation of double bounds of polymers and copolymers of conjugated dienes under mild reaction conditions.
Numerous catalysts are known for the hydrogenation of compounds containing unsaturated double bonds, catalysts which may be classified into two groups:
(1) Heterogeneous catalysts, generally consisting of a metal such as Ni, Pd, Pt, Ru, etc. deposited on a support such as carbon, silica, alumina, calcium carbonate, etc.; and PA1 (2) Homogeneous catalysts such as (a) Ziegler catalysts consisting of a combination of an organic salt of Ni, Co, Fe, Cr, etc., and a reducing agent such as the organoaluminum compounds and the like, and (b) organometallic compounds of Ru, Rh, Ti, La, etc. PA1 (1) By contacting the hydrogenated solution with a polar solvent such as acetone, methanol and the like, which, by being a poor solvent of the polymer, causes the latter's precipitation and permits its physical separation. PA1 (2) By contacting the hydrogenated solution with water and steam and eliminating the solvent by evaporation, separating the water and drying the polymer. PA1 (3) By direct evaporation of the solvent.
Heterogeneous catalysts are widely used in industry, but compared with the homogeneous catalysts they are less active and hence, in order to carry out the desired hydrogenation with these heterogeneous catalysts, large quantities of catalyst are needed and the reaction must be carried out at relatively high pressures and temperatures. The homogeneous catalysts are generally more active; a small amount of catalyst is sufficient, and the hydrogenation reaction can be carried out under milder pressure and temperature conditions. Nevertheless, homogeneous catalysts have the disadvantage that their stability is not sufficient and the separation of the catalyst or its decomposition products from the hydrogenated products can be complicated and costly.
Polymers of conjugated dienes such as 1,3-butadiene and isoprene and the copolymers of these dienes with vinylaromatic monomers, e.g. with styrene, are widely used in industry as elastomers. These polymers contain double bonds in their chain, which permit their vulcanization, but whose presence causes a low resistance to aging and oxidation. Some block copolymers of conjugated dienes and vinylaromatic hydrocarbons are used without vulcanization as thermoplastic elastomers, as transparent impact-resistant resins, or as modifiers or compatibilizers of polystyrene and polyolefin resins. However these copolymers have a low resistance to aging and oxidation by atmospheric oxygen and by ozone, due to the presence of double bonds in their chain. Hence the use of these copolymers in applications requiring exposure to the external environment is limited. The resistance to oxidation by oxygen and ozone, and, in general, the resistance to aging, may be considerably improved by hydrogenating these polymers to obtain total or partial saturation of the double bonds. Numerous processes have been proposed for the hydrogenation of polymers which contain olefinic double bonds. Two types of processes are generally involved: those which use the aforementioned supported heterogeneous catalysts, and those using homogeneous catalysts of the Ziegler type or organometallic compounds of rhodium and titanium.
In the processes using supported heterogeneous catalysts, the polymer to be hydrogenated is first dissolved in a suitable solvent and then contacted with hydrogen in the presence of the heterogeneous catalyst. The contact of the reactants with the catalyst is difficult due to the relatively high viscosity of the polymer solution, to steric hindrances of the polymer chain, and to the high adsorption of the polymer which, once hydrogenated, tends to remain on the surface of the catalyst, interfering with the access to the active centers of the nonhydrogenated polymer. Hence, to achieve complete hydrogenation of the double bonds, large quantities of catalyst and severe reaction conditions are required, which usually cause decomposition and gelification of the polymer. Furthermore, in the hydrogenation of copolymers of conjugated dienes with vinylaromatic hydrocarbons the aromatic nuclei are also hydrogenated, and it is difficult to effect a selective hydrogenation of the double bonds of the polydiene units. Likewise, the physical separation of the catalyst from the solution of hydrogenated polymer is extremely difficult, and in some cases a complete elimination is impossible due to the strong adsorption of the polymer on the heterogeneous catalyst.
In processes using Ziegler-type catalytic systems the reaction takes place substantially in a homogeneous medium, and hence the hydrogenation may be carried out under mild pressure and temperature conditions. Moreover, by adequately selecting the conditions of hydrogenation it is possible to selectively hydrogenate the double bonds of the chains of the copolymers of conjugated dienes and vinylaromatic hydrocarbons without hydrogenating the aromatic rings. Nevertheless the elimination of the catalyst residues from the reaction products--which is absolutely necessary because these residues have an unfavorable effect on the stability of the hydrogenated polymers--is a complicated and costly step. Other processes using other homogeneous catalysts, e.g. the rhodium compounds described in U.S. Pat. No. 3,898,208 and in the patent JP 01.289,805 have the disadvantage of the high cost of the rhodium catalysts.
It is known that hydrogenation catalysts in which one of the components is a derivative of cyclopentadienyltitanium (U.S. Pat. No. 4,501,857) are used--necessarily in the presence of organolithium compounds--for the hydrogenation of the olefinic double bonds of the polymers of conjugated dienes. U.S. Pat. No. 5,039,755 described the hydrogenation of polymers that had been synthesized by means of an organolithium compound and which have been completed by the addition of hydrogen, the presence of the lithium hydride formed in the final reaction being necessary in this case. The examples of both patents use the compound Cp.sub.2 TiCl.sub.2, which is not soluble in inert organic solvents and which has a tendency to hydrolize, thus forming hydrogen chloride which may cause corrosion of equipment.
Likewise, in British Patent Application No. 2,159,819 A, it is indicated that species of the Cp.sub.2 TiR.sub.2 type (R=alkylaryl groups) are catalysts capable of selectively hydrogenating the double bonds of polymers and copolymers of conjugated dienes, without requiring the presence of an organolithium compound.
Finally, European Patent No. 434,469 A2 describes the use of an extraordinary complex catalytic system, comprising a biscyclopentadienyltitanium compound in combination with an organometallic compound of aluminum or magnesium and alkaline metals in the presence of alkoxides of alkaline metals and polar compounds of ether, keton, sulfoxide, etc. type. Said catalytic system is capable of hydrogenating the double bonds of polymers and copolymers of conjugated dienes.
To obtain more economical hydrogenation process, present-day industry feels the need of having homogeneous catalysts available which are more effective than those currently known, which are stable, simple and active in concentrations that are sufficiently low so as to be able to avoid the costly step of elimination of catalyst residues form the hydrogenated polymers.